PCD inserts are used to form the cutting tips on underground drill bits, such as those used to drill oil and gas wells. Such inserts are cylindrical in nature, having a substrate which is typically sintered carbide and a layer of sintered polycrystalline diamond on an end of the cylinder. Multiple of such inserts are attached to drill bits as the PCD forms a durable cutting edge.
One limitation in the use of PCD cutting tips is the solvent metal which occupies the interstitial spaces between the diamond crystals. The diamond accounts for about 85 to 95 percent of the PCD, and the remaining material is a metal which acts as a solvent for carbon and a catalyst for diamond formation while sintering the PCD. The fraction of solvent metal is sufficient to cause problems in using the resulting PCD cutting insert. One problem is that the solvent metal expands more with temperature than diamond, and can cause cracking of the PCD layer as the cutting insert is used. Another limitation is that the solvent metal, being a solvent for carbon during the formation of diamond crystals, also acts as a carbon solvent for the degradation of the diamond at elevated temperatures. As such, the solvent metal remaining in the PCD causes the diamond to convert into carbon dioxide, carbon monoxide, or graphite at temperatures near 700 degrees Celsius.
As such, it is desirable to remove the solvent metal from the PCD cutting inserts before use. The solvent metal may be etched from the PCD using a mixture of strong acids, such as hydrofluoric and nitric acids (HF and HNO3). U.S. Patent Publication 2007/0284152 discusses the use of PCD cutting inserts, the problems associated with the solvent metal remaining in the PCD, and the etching of the PCD in acid to remove the solvent metal. In removing the solvent metal from the sintered diamond with acid, it is necessary to protect the substrate from the acid, as it is not desirable to etch or erode the substrate.
U.S. 2007/0284152 shows a fixture in FIG. 12 which is used to hold the PCD insert during etching and to protect the substrate from the acid. For discussion, the fixture is reproduced as Prior Art FIG. 2. FIG. 1 shows a typical PCD cutter insert 10. The insert 10 includes a substrate 14 and a PCD layer 18. As discussed, the substrate 14 is typically sintered carbide, which is comprised of metal carbides sintered together by metals. The PCD layer 18 typically includes about 85 to 95 percent diamond crystals and the remainder an appropriate solvent catalyst metal. The insert 10 is typically about 0.5 inches in diameter and about 0.75 inches in length. To increase the useful life of the insert 10, it is desirable to remove the solvent metal from between the diamond crystals.
FIG. 2 shows a cross-sectional view of a prior art fixture 22 used to hold the insert 10 in order to acid etch the PCD layer 18 to remove the solvent metal from between the diamond crystals. The fixture 22 has a center bore 26 which receives in insert 10, a hole 42 connecting the center bore through the back side of the fixture, and a groove 34 formed adjacent the front of the center bore. In use, the insert 10 is placed into the center bore 26 of the fixture 22. Afterwards, an elastomeric o-ring 30 is placed into the O-ring groove 34 formed in the front part of the bore 26. The insert 10 is then slid out of the bore 26 into the position shown, causing the o-ring 30 to seat on the diamond layer 18. A rubber stopper 38 is then placed into the hole 42 formed in the back of the fixture 22. The insert 10 is thus sealed into the fixture 22, having only a portion of the diamond table 18 exposed for etching. Etching is accomplished by placing the fixture 22, with the diamond table 18 facing downwardly, into a shallow bath of concentrated acid. The acid bath is kept at a desired temperature for a desired time period. Typically, the inserts 10 are etched for a period of 5 to 10 days in order to remove the solvent metal to a sufficient depth.
There are several problems associated with the fixtures 22 of FIG. 2. One significant problem is the expense of the fixture 22. The o-ring groove 34 must be machined into the fixture 22, making the cost of the fixture about $4.00 each. Since the fixtures typically may be used only a few times, the cost per insert etched is high. Another problem with the fixtures 22 is the time required to load the insert 10 into the fixture. Multiple steps are required to load the insert 10, install the o-ring, and set the insert at the proper depth. This increases the time required for assembly prior to etching, raising the cost of etching the insert 10.
Additionally, the O-ring 30 itself also presents a weakness in the design. Since the O-ring is elastomeric, it can be nicked or damaged while pushing the diamond table 18 through the o-ring during installation. Damage to the o-ring often results in a failed seal and thus an insert which is damaged during etching. Additionally, the O-ring 30 itself adds significant cost to the procedure, since the O-ring costs about $0.50, and is replaced after each use. Even using an O-ring 30 properly selected for the acids, such as a Viton® o-ring, the o-ring periodically fails while etching, resulting in a damaged part. Even if the o-ring 30 does not fail, it is typically softened by the acid and must be laboriously removed from the PCD insert 10 after etching.
A final limitation of the fixture 22 is the inability to precisely delineate the etched and non-etched portions of the diamond layer 18. FIG. 3 illustrates an etched PCD insert 10a. The o-ring 30 and fixture 22 produce an irregular border between the non-etched diamond layer 18 and the etched portion of the diamond layer 18a. The irregular boundary between the etched and non-etched portions of the diamond layer 18 require conservative placement of the insert 10 in the fixture 22 so as to prevent etching of the substrate 14. Additionally, an irregular boundary between etched and non-etched diamond layer 18 may result in damage to or failure of the insert 10 at the portions of the diamond layer 18 which still have solvent metal therein.
There is thus a need for an improved fixture for etching PCD drilling inserts. There is a need for an etching fixture which is easier to use, more reliable, and less expensive than prior art fixtures.